The present invention relates to an occupant weight detection system based on the fluid pressure in a bladder placed in or under a foam seat cushion, and more particularly to compensation of the detected weight for aging and usage of the foam seat cushion.
Vehicle occupant detection systems are useful in connection with air bags and other pyrotechnically deployed restraints as a means of judging whether, and how forcefully, to deploy the restraint. One fundamental parameter in this regard is the weight of the occupant, as weight may be used as a criterion to distinguish between an adult and an infant or small child. A known and particularly effective way of estimating occupant weight is by measuring the pressure in a fluid-filled bladder disposed in or under the foam seat cushion. See, for example, the U.S. Pat. No. 5,987,370 to Murphy et al. and U.S. Pat. No. 6,138,067 to Cobb et al., both of which are assigned to the assignee of the present invention, and incorporated herein by reference. In general, the measured pressure in such systems increases substantially monotonically from a xe2x80x9cvacant seatxe2x80x9d pressure as occupant weight is applied to the seat cushion. Significantly, the aforementioned U.S. Pat. No. 6,138,067 to Cobb et al. describes how the vacant seat pressure tends to shift with age and usage of the seat, and discloses a system for adaptively adjusting a calibrated value of the vacant seat pressure if the measured pressure falls below the calibrated value for at least a predetermined period of time.
Further development has additionally revealed that age and usage of the seat tends to shift the pressure vs. weight relationship of an occupied seat. Thus, even if the vacant seat pressure is properly calibrated, the estimated occupant weight will vary depending on the age and usage of the seat. Unfortunately, the adaptive technique used to adjust the vacant seat pressure in the aforementioned U.S. Pat. No. 6,138,067 to Cobb et al. cannot be used to compensate for changes in the pressure vs. weight relationship of an occupied seat, since the weight used to calibrate the pressure vs. weight relationship only occurs in a factory setting. Accordingly, what is needed is a pressure-based occupant weight detection system that continues to accurately detect occupant weight in spite of aging and usage of the seat.
The present invention is directed to an improved occupant weight estimation system in which the pressure in a fluid-filled bladder disposed in or under a foam seat cushion is measured as an indication of occupant weight, and wherein the pressure vs. occupant weight relationship is periodically adjusted over time to statistically compensate for changes due to aging and usage of the foam seat cushion. According to the invention, the system controller develops an aging adjustment value based on a measure of the cumulative aging and usage of the seat, and uses the developed adjustment value to compensate the operation of the system. The aging adjustment value is determined primarily as a function of occupant weight and time of seat occupancy, and the compensation is achieved by using the aging adjustment value to adjust either the estimated weight, or a threshold to which the measured pressure is compared for purposes of deciding if restraint deployment should be enabled. In the illustrated embodiment, the aging adjustment value is limited by a learned reduction of the vacant seat pressure.